The irreversibility field of multifilamentary Nb-Ti wires was measured and found to be seriously suppressed due to the flux creep at high temperatures while the critical current density was fairly large. This is explained with the small pinning potential as compared with a bulk specimen, since the pinning potential is limited by the size of the filaments. The experimental result is compared to the prediction based on the flux creep theory. Although the degradation of the irreversibility field is theoretically explained by the mechanism of flux creep, there exists a quantitative difference in the vicinity of the critical temperature. A theoretical analysis assuming a distribution of the critical temperature gave satisfactory results when compared to the experimental results.